How to make a corner and close a room

A wall from the end of another wall. Place the square behind the free end of the wall and drag to the side. You get a corner, and the inner length of the original wall does not change.
A wall between the ends of two walls. Start from the square behind the end of the first wall and bring the end to the end of the second. The end will also sit behind the end of the wall, the wall will lie straight, and both corners will be the same.
A wall flush with the end. If you start from the edge of a wall right at its end, the new wall will run flush with the end. When brought to the end of another wall, it will also finish flush, and that wall will not get longer.